Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.

Mobilizon - A free and federated tool to get our events off Facebook

Galette - Galette is a membership management web application towards non profit organizations.

Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.

Bigtent - BigTent is a web-based organization and communication portal for groups.

DownToMeet - DownToMeet is a platform to organize, find, and attend group events.